(1) Field of the Invention
The present invention relates to an image rotation processing method and, more particularly, to a signal processing method for transforming an image represented on two-dimentional orthogonal coordinates into an image rotated by an arbitrary angle, that is to say, a high-speed processing method which is suitable for editing documents including images and for matching seal impressions.
(2) Description of the Prior Art
Consideration will be taken into the process of rotating a digital image, which is defined as a set of image elements in an orthogonal lattice, by an angle of .theta. radians. The image rotation is a kind of affine transformations and is defined by the following equation: ##EQU3## wherein ##EQU4## designates the image element coordinates of an original image, and ##EQU5## designates the image element coordinates of the rotated image.
The image rotation method of the prior systems generally repeats the coordinate transformations of the Equation (1) for each image element and finds it difficult to conduct the rotations at a high speed. (for example, the methods disclosed in: (1) Japanese Patent Application Laid-Open No. 57-117061; (2) Literature Study Papers IE 78-12 by the Institute of Electronics and Communication Engineers of Japan; and (3) Computer, IEEE, p.p. 24-26, June 1983) We, the inventors, have invented an image rotation method (Japanese Patent Application No. 57-176151) for transferring adjoining image elements as a whole. This method can have an arbitrary angle of rotation but has a defect in that its processing time depends upon the rotational angle such that it takes the maximum for the rotation of (2n-1).times..pi./4 radians (1.ltoreq.n.ltoreq.4).